6.8.3.3 Mineral dust aerosols

Levin et al. (1996) observed desert dust particles coated with sulphate. Such
particles may originate from in-cloud scavenging of interstitial dust particles
followed by evaporation of the cloud droplets, condensation of SO2 onto dust
followed by oxidation, or even coagulation of dust and sulphate particles. The
presence of soluble material (which may be of anthropogenic origin) on the desert
dust particles converts them into large and effective CCN which may affect the
cloud microphysics. Whether this effect results in a significant climate forcing
has not been investigated and cannot presently be quantified.

6.8.3.4 Effect of gas-phase nitric acid

Kulmala et al. (1993, 1995, 1998) argued that enhanced concentrations of condensable
vapours (such as HNO3 and HCl) in the atmosphere could affect cloud properties
by facilitating the activation of cloud condensation nuclei. The impact of such
an effect on the planetary cloud albedo has not been assessed.

6.8.4 Indirect Methods for Estimating the Indirect Aerosol
Effect

6.8.4.1 The "missing" climate forcing

Hansen and colleagues have used two alternative approaches to characterise
and quantify any "missing" climate forcing besides that due to greenhouse
gases, solar constant, O3, and aerosol direct effect. Hansen et al. (1995) used
a simplified GCM to investigate the impacts of various climate forcings on the
diurnal cycle of surface air temperature and compared them with observations.
They found that, although the aerosol direct effect or an increase in continental
cloud albedo could contribute to damp the surface temperature diurnal cycle,
only an increase in continental cloud cover would be consistent with observations
(Karl et al., 1993). The required cloud increase depends on cloud height and
would be of the order of 1% global coverage for low clouds (i.e., 2 to 5% over
land). We cannot rule out that such a change is an unidentified cloud feedback
rather than a forcing. Hansen et al. (1997b) also argued that agreement between
observed and computed temperature trends requires the presence of another forcing
of at least -1 Wm-2 which is inferred as being due to the indirect effect. In
their calculations, the direct tropo-spheric aerosol effect does not play a
large net role, because the moderately absorbing aerosol assumption leads to
an offset between its sunlight reflecting and absorbing properties insofar as
the top of the atmosphere irradiance change is concerned. However, this method
assumes that the observed change in temperature since pre-industrial times is
primarily a response to anthropogenic forcings, that all the other anthropogenic
forcings are well quantified, and that the climate sensitivity parameter (Section
6.1) predicted by the GCM is correct (Rodhe et al., 2000). Therefore it may
simply be a coincidence that the estimate of Hansen et al. (1997b) is consistent
with the GCM estimates discussed above.

6.8.4.2 Remote sensing of the indirect effect of aerosols

Han et al. (1994) analysed AVHRR satellite radiances to retrieve the cloud
droplet size of low-level clouds. They reported significant inter-hemispheric
differences for both maritime and continental clouds. Boucher (1995) showed
that, if this difference is to be attributed to anthropogenic aerosols, it implies
a differential forcing of about -1 Wm-2 between the two hemispheres. Assuming
a Northern Hemisphere to Southern Hemisphere ratio of 2:1 for the aerosol indirect
effect, this would imply a globally-averaged forcing of -1.5 Wm-2. It is not
clear, however, to what extent changes in cloud droplet size are related to
change in aerosol concentrations. For instance, Han et al. (1998) showed that
cloud albedo decreases with decreasing droplet size for the optically thinner
clouds over the oceans. While this does not invalidate the aerosol indirect
effect at all, it underlines the limitations in using satellite observed changes
in droplet size to compute the aerosol indirect forcing. Therefore it seems
difficult at present to use satellite observations to estimate the first aerosol
indirect forcing unless some changes in cloud albedo could be tied to changes
in aerosol concentrations under the assumption of constant liquid water content.
Satellite observations do play, however, a key role for evaluating models of
the indirect aerosol radiative effect (Ghan et al., 2001b).